The Question:

Why are there 12 men on a Canadian football team?

The Answer:

Although both the National Football League and the Canadian
Football League require seven men to be on the line of scrimmage, the
CFL allows five (to the NFL's four) players in the backfield. All
five, plus the players lined up at each end of the line of scrimmage,
are eligible receivers. This formation is easier to accomplish in the
CFL because regulation fields are 15 yards wider then NFL
fields.

It's just one of the several differences between the two sports.
To learn more about the CFL check out their web site.